Syria announced a fresh, major offensive against besieged rebel-held eastern Aleppo on Thursday, as foreign ministers met and failed again in New York to agree on any diplomatic plan to try to stop the war.

U.S. Secretary of State John Kerry emerged from the meeting, held in a Manhattan hotel, saying he was "frustrated," but that he did not want to close the door on the cease-fire plan for Syria he brokered earlier this month in Geneva with his Russian counterpart.

"The United States will continue to pursue every avenue of progress that we can because it is the only way to stop the killing. It's the only way to ease the suffering and it's the only way to make possible the restoration of a united Syria," Kerry said. If the process fails, he warned, "this catastrophic situation is going to get even worse."

In this photo provided by the Syrian Civil Defense White Helmets, rescue workers work the site of airstrikes in al-Mashhad neighborhood in the rebel-held part of eastern Aleppo, Syria, Sept. 21, 2016.

Kerry made his comments to reporters, from whom he took no questions, after the International Syria Support Group (ISSG) convened for a second time this week in New York City amid increasing pessimism about the fate of Syria.

'Only path forward'

The U.S. secretary of state said there was near unanimity in the ISSG meeting "that this process is the only viable path forward."

U.S. officials who were in the room, speaking on condition they not be named, described Thursday's 2½-hour meeting as "pretty contentious." And the United Nations' Syria mediator, Staffan de Mistura, said it was "long, painful and disappointing."

Russia's foreign minister, Sergei Lavrov, emerging from the multinational discussion, said "nothing happened" in the talks.

Kerry said the U.S. "exchanged ideas with the Russians, and we plan to consult [Friday] with respect to those ideas"; if the Russians come back with constructive proposals, he added, "We will listen." But a senior U.S. official said no meeting is scheduled for Friday and "the ball is very much in the Russians' court."

Discord over airstrikes

Another senior American official characterized the diplomacy as now approaching "a climactic stage" and declined to speculate on what would happen if there is no meeting of minds between U.S. and Russian diplomats.

A man carries an injured child after airstrikes on the rebel-held al-Qaterji neighborhood of Aleppo, Syria Sept. 21, 2016.

French Foreign Minister Jean-Marc Ayrault demanded that planes attacking Syrian civilians be grounded. He said the response from Damascus' top ally, Russia, "was not satisfying."

Iran, another strategic ally of Syria, also opposes a cessation of airstrikes.

"If you ground flights, you are aiding the terrorists," Iranian President Hassan Rouhani told reporters at a hotel across the street from the United Nations.

Analysts' pessimism deepens

Hours earlier, at a congressional hearing in Washington, Senate Armed Services Committee Chairman John McCain said sarcastically that President Barack Obama had sent his "intrepid but delusional secretary of state to tilt yet again at the windmill of cooperating with [Russian President] Vladimir Putin."

But U.S. Defense Secretary Ash Carter defended Kerry before the committee for "working so tirelessly to see an arrangement which, if implemented, would ease the suffering."

Analysts are increasingly pessimistic.

"As conditions on the ground in Syria take another downward turn and as diplomatic attempts to rescue the U.S.-Russia agreement take an increasingly strained stance, I've chosen to take a critical look back," Middle East Institute senior fellow Charles Lister told VOA.

"The U.S. approach to tackling the Syrian crisis has resolutely failed, and our insistence on working with troublesome actors while focusing on symptoms rather than root causes is leading us further down a dangerous path," Lister said.

The ISSG is led by Russia and the United States, whose top diplomats on Wednesday presented sharply contrasting views at the United Nations about Monday's lethal attack on a 31-truck aid convoy into Syria from Turkey.

The attack, which one U.N. official said could be considered a "war crime," prompted a temporary suspension of aid shipments into Syria and threatened to destroy the tenuous cease-fire devised by Moscow and Washington.

The United States says Russian or Syrian Su-24 planes hit the U.N. convoy as it was unloading supplies at a warehouse west of Aleppo. The Russians have denied any responsibility, while Syrian President Bashar al-Assad, in an interview with The Associated Press, blamed "militants" and "terrorists."

Attempts to deliver aid resumed Thursday with U.N. trucks carrying food, medicine and other supplies for 35,000 people arriving in a rebel-held suburb of Damascus.

U.N. humanitarian adviser Jan Egeland is appealing to Assad to "do your bit to enable us to get to eastern Aleppo and also the other besieged areas."

Egeland noted that the U.N. also must obtain "assurances in the east Aleppo case from the armed opposition groups" fighting against Assad.

Risks from all sides

U.N. aid coordinators hope to deliver relief soon to the rebel-besieged towns of Foua and Kufreya in the Idlib governorate and government-blockaded Madaya and Zabadani, near the Lebanese border, Egeland said.

Staffers of the Syrian Arab Red Crescent assist families of gunmen as they leave from Al-Waer, the last rebel-held neighborhood of Homs, Syria, Sept. 22, 2016.

Under the Geneva cease-fire agreement, if there are seven days of relative calm and aid is able to reach critical communities, then the U.S. and Russian militaries are to begin coordinating separate but synchronized air attacks against Islamic State and Nusra rebels. It would be the first time the two countries have worked together in a military conflict since World War II.

U.S. military officials have been reluctant to cooperate with the Russians because of their crude bombing techniques, which result in significant civilian casualties.

"I do not believe it would be a good idea to share intelligence with the Russians," Marine General Joseph Dunford, chairman of the U.S. Joint Chiefs of Staff, told the Senate Armed Services Committee on Thursday, indicating any U.S.-Russian coordination would be very limited.

The United States and many other countries assert there is not a military solution to the conflict and that Assad cannot stay in power. Russia is a longtime and staunch supporter of Assad, as is Iran.

The ISSG, composed of 23 nations plus the Arab League, European Union, Organization of Islamic Cooperation and the United Nations, is dedicated to finding a diplomatic solution to the civil war in Syria, which began almost six years ago. However, a peaceful resolution is looking increasingly dubious, and the world's worst refugee crisis in 75 years — spawned by the civil war — is now predicted to become an even greater problem.

Nike Ching and Jeff Seldin in Washington and Margaret Besheer at the United Nations contributed to this report.